Familial Hypertrophic Cardiomyopathy (FHC) is an autosomal-dominant disease resulting from mutations in genes encoding sarcomeric proteins. Cardiac arrhythmias and sudden death are a major cause for the high mortality of patients with sarcomeric mutations. While there is a rough relationship between prognosis and the degree of cardiac hypertrophy and fibrosis, this genotype/phenotype correlation is weak. In particular, the mechanism(s) that cause ventricular arrhythmias in young patients with either limited or absent myocardial hypertrophy or fibrosis are poorly understood. In vitro studies have shown that most FHC-linked mutations in sarcomeric proteins (e.g., troponin T, troponin I, tropomyosin) increase myofilament Ca2+ sensitivity. Our work during the last funded period strongly suggests that increased Ca2+ sensitivity in and of itself may be pro-arrhythmic: Transgenic mice expressing Ca2+- sensitizing mutations of troponin T (TnT-I79N, TnT-F110I) have an increased incidence of ventricular arrhythmias that occur in the absence of myocardial hypertrophy or fibrosis. In contrast, transgenic mice expressing the relatively benign FHC-linked TnT-R278C mutation, which does NOT increase Ca2+ sensitivity, do NOT develop ventricular arrhythmias. Further supporting this hypothesis we showed that acutely increasing myofilament Ca2+ sensitivity with EMD57033 in wild-type animals also increased the risk for ventricular tachycardia (VT). Moreover, myofilament de-sensitization and contractile uncoupling prevented the pro- arrhythmic effects of the TnT-mutants and EMD. Based on these data we found that EMD exerted its effect by duplicating the altered myocyte Ca2+ handling, action potential remodeling, afterdepolarizations and ventricular arrhythmias seen in transgenic mouse hearts expressing Ca2+-sensitizing TnT mutants. Thus, we hypothesize that increased myofilament Ca2+ sensitivity contributes to the risk for ventricular arrhythmias. Each AIM will examine specific mechanisms that could contribute to this chain of events. In addition to studying mice expressing FHC-linked mutations, we will also use Ca2+ sensitization by EMD to model effects of sarcomeric mutations in rabbits to test the hypothesis in a model with more human-like cardiac electrophysiology. AIM 1: To determine the effect of myofilament Ca2+ sensitization on cardiac Ca2+ handling AIM 2: To determine the effect of myofilament Ca2+ sensitization on the ventricular action potential and its heart rate dependence AIM 3: To determine the effect of myofilament Ca2+ sensitization on cell-cell coupling and on arrhythmia susceptibility in the intact heart The results of the proposed experiments will significantly advance our understanding of arrhythmia mechanisms responsible for the high rate of sudden cardiac death in sarcomeric cardiomyopathies. PUBLIC HEALTH RELEVANCE: Familial hypertrophic cardiomyopathy is the most common inherited heart disease associated with heart enlargement and a high mortality rate from abnormal heart rhythms (=arrhythmias). The proposed research focuses on discovering the underlying mechanism(s) responsible for the arrhythmias. Result from our studies may provide new therapeutic strategies for this presently incurable disease.